Scott216 wrote:Tech support told me that as long as the input resistance is 10,000 ohms or more, the CT accuracy will be fine. The output voltage rating is RMS. It outputs 0.33 volts when 50 amps is going through the CT. The CT will clip the voltage at 0.5 volts if you put more then 50 amps through the CT.
I had a somewhat detailed response that got poofed (that's 2x in as many days) when the SF forum went offline. Because there's only so much time in a day let me ask what error are you willing to tolerate ? Are you looking to have the system report a current measurement thats with +/-1% of the true current ? +/-5% ?? +/-?? % ???
Over what range of currents do you want this level of accuracy ? Because of the diodes, small current levels will have more error than larger levels. You can mitigate this somewhat by having a large gain such that the range of levels with "bad" accuracy is reduced. Just as an example, would you care that 0.2 A RMS as reported was really 0.3 A RMS ? What the lower limit that you need to have the above desired level of accuracy for ?
Is there an upper current limit that you need to measure ? By increasing the gain of the circuit, you can get better accuracy (and resolution from the Xbee A/D) but at the cost of dynamic range. Specifically the circuit may clip before the 50 A RMS output of the CT (0.333 V RMS) is reached. Would you care if the max reading was limited to ... say 30 A RMS ? And that any level above that would still be reported as 30 A RMS ? IOW do you want/need the full range up to 50 A RMS or do you just need to know something has gone wrong and the motor is probably stalled ? What level of current does that for you ?
Just to toss out some numbers ... As it stands as posted above the circuit needs to filter out the "humps". If you put a low pass filter (LPF) with a bandwidth (BW) of ~ 1 Hz (that's the 166 msec time constant I mentioned above), any ripple content at 120 Hz component will be reduced by a factor of 200. Now perhaps that's overly conservative (I'll ponder it some more tonight) and a higher BW for the LPF would be OK. To do a LPF you'd put a capacitor in parallel with R5. The cap value would be 16.6 uF if R5 remains at 10K (I don't think it should) to get the 1 Hz BW. Larger R5 values would reduce the cap value needed and increase the gain (which is also needed).
The gain (Vout DC/Vin RMS) of the LPF equipped circuit is nominally 1.414*(2/pi) after rectification and filtering. A 50 A RMS level would result in a 0.333*1.414*2/pi = 300 mV, obviously too low a voltage so more gain in both the 1'st op-amp stage and 2'nd is needed. To prevent unintentional clipping in the 1'st stage the additional gain should be distributed. If you're running this circuit off the 3.3 VDC you're powering the Xbee with (are you ?) then the best you could hope for is a max 3 VDC output. If you want to retain the full range to 50 A RMS that means a gain increase by a factor 10 (easy to do). Increase R2, R3 and R4 ... I can figure out the right amounts when you tell me what the desired dynamic range is. I do have to think about how to connect your CT given the circuit needs to run off a single ended supply voltage (as shown it was running off +V and -V supplies).
Lastly is there going to be a computer or some ability to "massage" the digital data coming from the Xbee before it's final usage (whatever that is) ? I ask because if so then perhaps a lookup table or some other form of error correction could be implemented. For example a reading of ABC counts could be calibrated to be XYZ A RMS. In this way the circuit could be built to "loose" tolerances for gain and perhaps not need offset/bias voltage correction. These could be done in software instead. Then again perhaps you don't need much in the way of accuracy ...
BTW is this a one off prototype thing or are you going to have a PCB made ?